Charging

[Ben sage]

 Hi guys I was wondering how this old car battery charger works  because it charges battery’s with AC  and also at the 12 V setting I measure 50 V I’ve tested it it does work

[Jeroen3]

As long as there is DC offset it is fine.

If there isn’t, the battery lifetime will be halved. (~2 years max)

But I suspect you‘re not probing right.

[Ben sage]

It’s not DC offset there is negative and positive and I put the ground clip to the negative and the probe to positive

[xrunner]

It’s not DC offset there is negative and positive and I put the ground clip to the negative and the probe to positive

You seem to be showing 50Vpp on the scope?

50 Vpp would be 25 Vpeak, and would then be 17.1 Vrms. I think the charger uses the 50Vpp into a full wave bridge, and then maybe a capacitor, so I don't think it's unreasonable. But as was said, where are you probing the 50 Vpp is the question. 

[Ben sage]

Here’s how I hook them up And this is what It looks like inside

[sleemanj]

The plate on the left of your picture looks like a single plate selenium rectifier (what we would now use a diode for), it may well not be doing much in the way of rectification any more.


You may find this of use:
http://anoldtechniciansworkbench.blogspot.com/2013/01/one-amp-battery-charger.html

[jmelson]

The plate on the left of your picture looks like a single plate selenium rectifier (what we would now use a diode for), it may well not be doing much in the way of rectification any more.

it may also be working fine, just too leaky to show up any DC on a 1 Meg Ohm scope input.  Try putting a load like a 20 Ohm resistor (value not at all critical) or a car turn signal bulb on it as a load and try the scope again.

Jon

[james_s]

That's exactly what I was going to suggest, put a load on it, IIRC those old selenium rectifiers are pretty leaky.

I would probably replace it with a modern diode if I were going to actually use the charger. If the selenium rectifier fails, which they sometimes do, you'll never forget that awful smell.

[Ben sage]

I just found out from my dad that we have two of these chargers and this is the broken one So the one I tested does work but not this one and thanks guys for the great comments

[james_s]

Replace the selenium rectifier with a sufficiently rated diode and that will fix it right up, there's not really anything else to go wrong.